Conventional monochromators use ruled diffraction surface gratings, which have fairly low angular dispersion. In such conventional monochromators, as light is passed from one ruled grating to the next and through a small aperture at the exiting end of the monochromator, the intensity of the filtered light leaving the system is substantially lower than that of the light entering the system in the same spectral band. Also, to improve spectral resolution when using weakly dispersive elements, conventional monochromators typically use spatial filtering, a technique where small apertures, or slits, are used to allow only the desired wavelength to pass through the monochromator. However, when spatial filters are used to improve spectral resolution, a portion of the light of the selected wavelength which enters the monochromator is blocked by the aperture at the output. Additionally, several feet of path length (long distances between optical elements) are used to improve spatial separation of wavelengths in a device with low angular dispersion.
Volume holograms (also known as volume holographic optical elements) consist of volume holographic diffraction gratings written in photosensitive material. Volume holograms accept one wavelength band of light, transmit or reflect that wavelength depending on their write geometry, and filter out all other wavelengths of light by only transmitting the desired wavelength band at a selected angle. The wavelength band that is accepted is controlled by the angle at which the hologram is positioned with respect to the incoming light. As this incident angle is changed, the wavelength band accepted also changes.